Reduce repetitive EventTarget subclassing

Source/core/events/EventTarget.h

Index: Source/core/events/EventTarget.h
diff --git a/Source/core/events/EventTarget.h b/Source/core/events/EventTarget.h
index 35d209af6112b87233bcd4e2af4e88490922d75a..ee9799bab116b2cd2175aabda7a7144a408a9aec 100644
--- a/Source/core/events/EventTarget.h
+++ b/Source/core/events/EventTarget.h
@@ -131,7 +131,7 @@ namespace WebCore {
         virtual ~EventTarget();
 
         virtual EventTargetData* eventTargetData() = 0;
-        virtual EventTargetData* ensureEventTargetData() = 0;
+        virtual EventTargetData& ensureEventTargetData() = 0;
 
     private:
         virtual void refEventTarget() = 0;
@@ -146,6 +146,14 @@ namespace WebCore {
         friend class EventListenerIterator;
     };
 
+    class EventTargetWithInlineData : public EventTarget {
+    protected:
+        virtual EventTargetData* eventTargetData() OVERRIDE FINAL { return &m_eventTargetData; }
+        virtual EventTargetData& ensureEventTargetData() OVERRIDE FINAL { return m_eventTargetData; }
+    private:
+        EventTargetData m_eventTargetData;
+    };
+
     // FIXME: These macros should be split into separate DEFINE and DECLARE
     // macros to avoid causing so many header includes.
     #define DEFINE_ATTRIBUTE_EVENT_LISTENER(attribute) \